In recent years, for an earth environmental problem due to an exhaust gas etc., improvement in fuel consumption due to weight saving of a vehicle body of a transport machine such as an automobile has been pursued. In addition, enhancement of safety at vehicle body collision of an automobile without inhibiting this weight saving as much as possible has been also pursued. For this reason, particularly, for a vehicle body structure of an automobile, application of an aluminum alloy material which is lighter and is also excellent in energy absorbability, in place of a steel material which has previously been used, is increasing. The aluminum alloy material referred herein is a generic name of a rolled plate material, an extruded material, and a forged material of an aluminum alloy.
For example, use of aluminum alloy plates such as Al—Mg—Si-base AA to JIS 6000 series (hereinafter, simply referred to as 6000 series) and Al—Mg-base AA to JIS 5000 series (hereinafter, simply referred to as 5000 series) is studied for panels such as an outer panel (outer plate) and an inner panel (inner plate) of panel structures such as a hood, a fender, a door, a roof, and a trunk lid of an automobile.
In addition, as an energy absorbing member or a reinforcing material such as a bumper reinforcing material (also referred to as bumper reinforcement, or bumper armature) or a door reinforcing material (also referred to as door guard bar, or door beam), for securing safety of vehicle body collision of an automobile, aluminum alloy extruded materials such as Al—Zn—Mg-base AA to JIS 7000 series (hereinafter, simply referred to as 7000 series) and the 6000 series alloy are used. Further, an aluminum alloy forged material of the 6000 series alloy is used in an underbody part of an automobile such as a suspension arm.
These aluminum alloy materials are necessarily used by joining with a steel material (steel member) such as a steel plate and a mold steel which are originally used widely, in a vehicle body of a normal automobile, unless a vehicle body is an all-aluminum automobile vehicle body. Therefore, when an aluminum alloy material is used in a vehicle body of an automobile (member in a combination of a steel material and an aluminum alloy material), it necessarily requires dissimilar joint of Fe—Al (joining between different kinds of metal members of iron-aluminum).
However, as a problem when this Fe—Al dissimilar joint is performed by welding, there is generation of an intermetallic compound layer (hereinafter, also referred to as reaction layer) between Fe and Al, which is high hardness and very brittle, at a mutual joint interface. For this reason, although apparently mutually joined, sufficient joining strength is not obtained in many cases, in a joined body of dissimilar metals, by welding of Fe—Al dissimilar joint, due to a cause of generation of the intermetallic compound layer.
Reflecting such a situation, previously, for joining of these joined bodies of dissimilar metals (joined body of different kinds of metal members), not only welding, but also joining using such as a bolt and a rivet jointly, or an adhesive in combination are performed, but there is a problem such as a troublesome joining operation and an increase in the joining cost.
Then, previously, regarding a method of welding of Fe—Al dissimilar joint, joining by efficient spot welding, which is widely used in joining of a vehicle body of a normal automobile, has been studied. For example, a method of inserting an aluminum-clad steel between an aluminum material and a steel material has been proposed. In addition, a method of plating or inserting a metal having a low melting point on the steel material side has been proposed. Further, a method of holding insulating particles between an aluminum material and a steel material, and a method of making irregularities on a member in advance have also been proposed. Further, a method of forming a uniform oxidized membrane by heating in the atmosphere after removal of an ununiform oxidized membrane of an aluminum material, and performing spot welding using two layers of a multiple-layered steel plate of aluminum-steel, in a state where the contact resistance of an aluminum surface is enhanced, has also been proposed.
Meanwhile, it is known that also on the steel material side, when an element easily forming an oxide such as Si, Mn and Al is added in order to enhance the strength of a steel plate, an oxide containing these Si, Mn and Al is generated on a surface of a base material. It is also known that an oxide containing these Si, Mn and Al inhibits adherability between a surface coating such as zinc plating and a steel plate. Further, on the other hand, it is also known that, when a thickness of an oxide film containing Si, Mn and Al is made to be in a range of 0.05 to 1 μm by subjecting a steel plate to pickle, adherability between a surface coating such as zinc plating and a steel plate, and spot weldability between steel plates are improved (see Patent Document 1).
However, according to these previous techniques, sufficient joining strength is not obtained in a joined body of dissimilar metals of welding-joined Fe—Al, under the joining conditions of efficient spot welding which is widely used in joining a vehicle body of a normal automobile. In other words, the spot welding condition for obtaining joining strength necessarily becomes troublesome, and this is not practical.
To the contrary, particularly, various techniques intending spot welding of a joined body of dissimilar metals, which is produced by joining a 6000 series aluminum alloy material that is widely used for an automobile vehicle body and a high-strength steel plate that has a tensile strength of 450 MPa or more, have been proposed.
For example, in Patent Documents 2 and 3, spot welding of a steel material and an aluminum alloy material having a limited plate thickness of 3 mm or less, in a form of laminating two or more steel materials, or in a form of holding a steel material between aluminum alloy materials has been proposed. In Patent Document 4, improvement in joining strength by defining a nugget area at a spot welding part or a thickness of a boundary reaction layer has been proposed. In addition, in Patent Documents 5 and 6, improvement in joining strength by defining a composition, thickness and area of each generated compound in detail, respectively, on the steel material side and the aluminum alloy material side at a welding interface has been proposed.
Further, in Patent Document 7, obtaining of high joining strength of a joined body of dissimilar metals, under the appropriate spot welding condition, by once removing the existing oxide layer on a steel plate surface, converting a newly generated outer oxide layer into an oxide having a Mn and Si composition at a specific ratio and, further, defining a ratio of occupation of an inner oxide containing Mn and Si at a total amount of 1 at % or more, which is present in a steel region at a depth from a steel body surface of this steel material to 10 μm or less, in a high-strength steel plate having a specific composition has been proposed. In this Patent Document 7, diffusion of Fe and Al at spot welding is suppressed by a newly generated outer oxide layer containing Si and Mn and an inner oxide layer directly beneath the steel body surface, thereby, excess generation of an Al—Fe-base brittle intermetallic compound layer at a joining interface is suppressed. Incidentally, in Patent Document 7, a welding procedure is not limited, joining of dissimilar metals by spot welding in Example 1, laser welding in Example 2, and MIG welding in Example 3 is performed, respectively, and thereby, a joint body of dissimilar metals is manufactured.    Patent Document 1: JP-A No. 2002-294487 gazette    Patent Document 2: JP-A No. 2007-144473 gazette    Patent Document 3: JP-A No. 2007-283313 gazette    Patent Document 4: JP-A No. 2006-167801 gazette    Patent Document 5: JP-A No. 2006-289452 gazette    Patent Document 6: JP-A No. 2007-260777 gazette    Patent Document 7: JP-A No. 2006-336070 gazette